Field of the Invention
This invention relates to injection devices and, in particular, but not exclusively, to autoinjectors.
Description of the Related Art
In autoinjectors a syringe or cartridge (both hereinafter referred to generally as a syringe) is mounted in a body and a drive mechanism is provided so that, when the device is held against the flesh and fired, the syringe is moved forwardly to urge the needle into the injection site and thereafter the syringe plunger is advanced to expel a dose. After the dose has been expelled it is common to make the device safe, either by retracting the syringe into the autoinjector body or to extend and lock out a needle shroud from the front of the body, in each case shrouding the needle in a shrouding passage. Prior to use, the needle is usually covered by a boot to keep it sterile to prevent evaporation of the drug, and to facilitate assembly of the autoinjector, with the boot being withdrawn forwardly off the needle to prepare for an injection. In such designs it is necessary to withdraw the boot down the shrouding passage and this places conflicting demands on the designer. In order to allow the boot to be withdrawn, the shrouding passage must be wide enough to accommodate the diameter of the boot and any elements that engage a clasp the boot to remove it. Other design considerations mean that the maximum diameter of the boot is often similar to the outer diameter of the syringe barrel which may typically be 5 mm to 11 mm. Conversely, safety considerations require that the bore of the passage be sufficiently narrow to prevent a small child from pushing a finger into the open end of the shrouding passage surrounding the needle and sustaining a needle stick injury. It is therefore highly desirable to have a reduced effective aperture in the shrouding passage that still allows the boot to be withdrawn. Although particular reference is made to designs in which a boot is withdrawn axially from the needle, some similar considerations apply in general where access to the needle is to be obstructed in instances where there is no boot.